The invention relates generally to acoustic radiation stress measurements and more specifically concerns a method and apparatus for quantitatively characterizing those properties of fluids and solids related to material nonlinearity.
Each material has a unique set of nonlinearity parameters that characterizes that material and which are related to the thermodynamic state of that material. The thermodynamic state is here generalized to include the degree of residual or applied stress, the atomic or molecular composition of the material, defect structures, and the state of material order or disorder. The nonlinearity parameter thus serves as an important and distinct measure of the material state that is both complementary and supplementary to acoustic velocity and acoustic attenuation measurements.
Operationally, the nonlinearity parameter is responsible for acoustic waveform distortion leading to acoustic harmonic generation (the harmonics of the fundamental driving frequency) and acoustic radiation-induced static strains in the material. The acoustic radiation induced static strains are a manifestation of the acoustic radiation stress in the material. The previous art has been to measure the nonlinearity parameters by means of a harmonic generation method. The harmonic generation method, however, is sensitive to phase cancellation artifacts in the transducer and cannot give unambiguous values for the sign of the nonlinearity parameters without the addition of a phase detection system which must be calibrated to a standard sample.
It is the primary object of this invention to provide a method and apparatus for measuring the nonlinear parameters of a material that characterize that material.
Another object of this invention is to provide a method and apparatus for quantitatively characterizing those properties of fluids and solids related to material nonlinearity without the addition of a phase detection system.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawing.